This is a proposal to continue detailed investigation of defective interfering particles (DI) of enveloped RNA viruses, and of their role in attenuating acute virus infections in vivo, and in promoting long term persistent infections of cells in vitro and in vivo. Oligonucleotide mapping and RNA sequencing studies on DI and virus RNA segments will be carried out to determine sequences involved in DI generation, replication and interference, and to allow direct biochemical demonstration of the presence of DI RNA sequences in cells and tissues. In addition, we will attempt to devise more sensitive and quantitative bio-assays for DI in tissues, and to develop DI cloning methods. We will continue to study the biological and biochemical changes occurring in virus and DI as persistently infected cells of different types are cultured for many years as carriers. (We will do oligonucleotide mapping of carrier virus RNA, carrier virus mRNA, carrier DI RNA and peptide mapping of carrier virus and wild type virus proteins as chronic infection evolves toward a more stable cell-virus relationship.) We propose to utilize the nude mouse as a convenient in vivo environment with which to carry out detailed studies of RNA virus persistence. We recently found that carrier cells transplanted into nude mice can maintain persistence indefinitely (at least for 6 months), and the infected cells can readily be recovered and quickly reestablished in cell culture for biochemical analysis. When VSV carrier cells are transplanted they form non-growing nodules instead of the rapidly growing tumors formed by uninfected BHK21 cells (or "cured" cells). We plan to study this.